ASP.NET (MVC) Outputcache and concurrent requests

Question

Let's say that, theoratically, I have a page / controller action in my website that does some very heavy stuff. It takes about 10 seconds to complete it's operation.

Now, I use .NET's outputcache mechanism to cache it for 15 minutes (for examle, I use [OutputCache(Duration = 900)]) What happens if, after 15 minutes, the cache is expired and 100 users request the page again within those 10 seconds that it takes to do the heavy processing?

1. The heavy stuff is done only the first time, and there is some locking mechanism so that the other 99 users will get the cache result
2. The heavy stuff is done 100 times (and the server is crippled as it can take up to 100 * 10 seconds)

Easy question maybe, but I'm not 100% sure. I hope it is number one, though :-)

Thanks!

Answer 1

Well, it depends upon how you have IIS configured. If you have less than 100 worker threads (let's say, 50), then the "heavy stuff" is done 50 times, crippling your server, and then the remaining 50 requests will be served from cache.

But no, there is no "locking mechanism" on a cached action result; that would be counterproductive, for the most part.

Edit: I believe this to be true, but Nick's tests say otherwise, and I don't have time to test now. Try it yourself! The rest of the answer is not dependent on the above, though, and I think it's more important.

Generally speaking, however, no web request, cached or otherwise, should take 10 seconds to return. If I were in your shoes, I would look at somehow pre-computing the hard part of the request. You can still cache the action result if you want to cache the HTML, but it sounds like your problem is somewhat bigger than that.

You might also want to consider asynchronous controllers. Finally, note that although IIS and ASP.NET MVC will not lock on this heavy computation, you could. If you use asynchronous controllers combined with a lock on the computation, then you would get effectively the behavior you're asking for. I can't really say if that's the best solution without knowing more about what your doing.

Answer 2

It seems to lock here, doing a simple test:

<%@ OutputCache Duration="10" VaryByParam="*" %>

protected void Page_Load(object sender, EventArgs e)
{
    System.Threading.Thread.Sleep(new Random().Next(1000, 30000));
}

The first page hits the a breakpoint there, even though it's left sleeping...no other request hits a breakpoint in the Page_Load method...it waits for the first one to complete and returns that result to everyone who's requested that page.

Note: this was simpler to test in a webforms scenario, but given this is a shared aspect of the frameworks, you can do the same test in MVC with the same result.

Here's an alternative way to test:

<asp:Literal ID="litCount" runat="server" />

public static int Count = 0;

protected void Page_Load(object sender, EventArgs e)
{
  litCount.Text = Count++.ToString();
  System.Threading.Thread.Sleep(10000);
}

All pages queued up while the first request goes to sleep will have the same count output.